Spondylolisthesis is a term used to describe when one vertebrae slips forward on the vertebrae below it. This usually occurs because there is a spondylolysis in the superior vertebrae. There are two main parts of the spine that keep the vertebrae aligned, which include the disc and the facet joints. When spondylolysis occurs, the facet joint can no longer hold the vertebrae back. The intervertebral disc may slowly stretch under the increased stress and allow the upper vertebra to slide forward. In the vast majority of cases, stretching of the intervertebral disc only allows for a small amount of forward slip.
Surgical treatment for spondylolisthesis needs to address both the mechanical symptoms and the compressive symptoms, if they are present. The goals of surgery are to remove pressure on spinal nerves (i.e., decompression) and to provide stability to the thoracic/lumbar spine. In most cases of spondylolisthesis, decompression should be accompanied by uniting one spinal vertebrae to the next (i.e., spinal fusion) with spinal instrumentation (i.e., implants that are often used to help aid the healing process).
In other cases, the spinal disc and/or vertebral bodies may be displaced or damaged due to trauma, disease, degenerative effects, or wear over an extended period of time. This displacement or damage often causes chronic back pain. In order to alleviate the chronic back pain, a spinal disc is removed, along with all or part of at least one of the neighboring vertebrae. An implant is then inserted to promote fusion of the remaining bony anatomy. The success of spinal fusion is limited, however, due to several factors. For example, the spacer or implant or cage used to fill the space left by the removed disc may not be strong enough to support the spine. Furthermore, the spacer must be able to remain in the position in which it is placed by the surgeon. The space must also be comprised of such a material to promote bony growth around the spacer and within the spinal region.